The present invention relates to a piezoelectric device having a piezoelectric film deposited by vapor phase deposition, and a liquid-ejecting head that utilizes the piezoelectric device.
In recent years, active R&D efforts are being directed to various kinds of devices that use the piezoelectric element, as exemplified by actuators, sensors and memory devices, and piezoelectric films that are deposited by vapor phase deposition such as sputtering are drawing particular attention as functional films capable of high performance. Such piezoelectric films are typically used in the actuator section of inkjet heads, micropumps and so forth.
It is conventionally known that piezoelectric films in an applied electric field are vulnerable to high temperature and humidity and that they (the piezoelectric element) will deteriorate upon exposure to the moisture in the air. Specifically, the presence of water increases the leakage current, occasionally causing a dielectric breakdown in the piezoelectric film. In addition, the constituent elements in the piezoelectric film may react with the water to cause deterioration in the film. Various measures are taken to suppress the above-described dielectric breakdown or deterioration in piezoelectric films due to water.
For example, JP 2003-291343 A describes a liquid-propelling head comprising a channel-forming substrate that defines a pressure-generating compartment communicating with a nozzle opening through which a liquid is ejected and a piezoelectric device provided on one side of the channel-forming substrate, with a diaphragm being interposed, which comprises a lower electrode, a layer of piezoelectric element, and an upper electrode, characterized in that a sealing substrate having a piezoelectric device holding section that seals space as it is secured in a region that faces the piezoelectric device is provided on the side of the channel-forming substrate where the piezoelectric device is provided, and that at least a part of the periphery of the piezoelectric device holding section of the sealing substrate is joined to the channel-forming substrate via a glass joining layer made of glass.
In the liquid-propelling head described in JP 2003-291343 A, the sealing substrate is joined to the channel-forming substrate via the glass joining layer made of glass such that the piezoelectric device is sealed within the piezoelectric device holding section of the sealing substrate; the reference states that because of this design, no external moisture as in the atmosphere will get into the piezoelectric device holding section, whereby breakdown of the piezoelectric device due to moisture is prevented.
JP 10-226071 A describes an inkjet recording head including a piezoelectric oscillator comprising a lower electrode formed on a surface of an elastic plate that constitutes a pressure-generating compartment communicating with a nozzle opening, a layer of piezoelectric element formed on a surface of the lower electrode, and an upper electrode formed on a surface of the layer of piezoelectric element in a region that faces the pressure-generating compartment, wherein the upper electrode is formed independently in association with a region that faces the pressure-generating compartment and wherein an insulator layer that covers an area extending from the periphery of the upper surface of the upper electrode to the lateral sides of the layer of piezoelectric element is formed in such a way as to leave a window in at least an area that serves as a connection to an electrically conductive pattern. The inkjet recording head described in JP 10-226071 A prevents deterioration due to moisture absorption by covering the layer of piezoelectric element with the insulator layer.
JP 10-226071 A also describes forming a window in the insulator layer except in part of the periphery of the upper surface of the upper electrode, the window being larger than the area of connection between the upper electrode and the electrically conductive pattern and accounting for the most part of the displacement region of the layer of piezoelectric element. JP 10-226071 A states that in the disclosed inkjet recording head in which the above-mentioned window is formed in the most part of the region of the layer of piezoelectric element where it is displaced in an applied electric field, the piezoelectric element is displaced by a larger amount per unit voltage than when no such window is formed.